To an increasing extent, touch-sensitive panels are being used for providing input data to computers, cell phones, electronic measurement and test equipment, gaming devices, etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers. The GUI may be fixed or dynamic. A fixed GUI may e.g. be in the form of printed matter placed over, under or inside the panel. A dynamic GUI can be provided by a display screen integrated with, or placed underneath, the panel or by an image being projected onto the panel by a projector.
There are numerous known techniques for providing touch sensitivity to the panel, e.g. by using cameras to capture light scattered off the point(s) of touch on the panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into the panel.
US 2004/0252091 discloses an alternative technique which is based on frustrated total internal reflection (FTIR). Diverging beams from two spaced-apart light sources is coupled into a panel to propagate inside the panel by total internal reflection. The light from each light source is evenly distributed throughout the entire panel. Arrays of light sensors are located around the perimeter of the panel to detect the light from the light sources. When an object comes into contact with a surface of the panel, the light will be locally attenuated at the point of touch. The location of the object is determined by triangulation based on the attenuation of the light from each source at the array of light sensors.
U.S. Pat. No. 3,673,327 discloses a similar technique also using FTIR in which arrays of light beam transmitters are placed along two edges of a panel to set up a grid of intersecting light beams that propagate through the panel by internal reflection. Corresponding arrays of beam detectors are placed at the opposite edges of the panel. When an object touches a surface of the panel, the beams that intersect at the point of touch will be attenuated. The attenuated beams on the arrays of detectors directly identify the location of the object.
The FTIR techniques described above suffer from being costly, i.a. since they require the use of a large number of detectors and often also a large number of light sources. Furthermore, they are not readily scalable since the required number of detectors/sources increases significantly with the surface area of the panel. Also, the spatial resolution of the panel is dependent on the number of detectors/sources. Still further, the energy consumption for illuminating the panel may be considerable and increases significantly with increasing surface area of the panel.